Most present day cellular communication networks are analogous to wired telephone networks in the sense that, in each network, each of the telephone subscriber stations in the network is identified by a telephone number unique to that station. In the course of a customer placing a call from that station, the identification number of the station (which is a cellular phone in the case of cellular communications) is automatically signaled to a central office to enable the party in whose name the station is listed to be billed for the call.
An alternative mode of billing for the call is usable when the customer has a personal charge account with the telephone company separate from the account for the station. That alternative mode is for the customer, when making the call, to identify (by voice or telephone key pad input), to a telephone operator or computer in the network, the number assigned to and identifying that customer's personal charge account, whereupon the cost of the call is billed to that account.
A problem encountered in recent years is that both such methods for identifying to what account a call should be billed are insecure when customers are placing calls from cellular telephone sets. That is, as such calls are being placed, unscrupulous persons have been able to learn the identification number of the particular cellular telephone set being used. Moreover, if the customer is charging the cost of the call to that customer's personal account, such persons have been able to learn the identification number of that personal account. Those unscrupulous persons have then sold those identification numbers to others who have used them to make large numbers of telephone calls, the costs of which are billed by the telephone company to the innocent customer. Such telephone fraud has been the source of yearly losses to the telephone companies and their customers on the order of millions of dollars.
To prevent such fraud from being practiced, changes have already been made in Europe (and are expected to be made soon in the United States). Those changes are (a) abandoning the practice of the assigning to cellular telephone sets of unique set identifying numbers for use for billing purposes, (b) using instead for such purposes only the identification number of the personal account of the customer placing the call, and (c) communicating that number to the base station from the cellular phone set by having the customer insert into a slot in the set a card carrying data representing the personal account number of the customer, and then having such data read out by the set to permit it to communicate that number in enciphered form to the base station. Cards of the sort just described are sometimes referred to herein as "portable data carriers" or "dam carrying (or carrier) cards". The following of the practices just outlined precludes unauthorized persons from learning the personal account number of the customer placing the call.
While the changes just described constitute a significant advance in the art of making cellular telecommunications more secure, their implementation creates in instances a mechanical difficulty of the following character. Most cellular telephone sets are apparatus large enough to have formed therein a slot for accommodating a data carrying card of standard size in the sense that it is of the same size as that used for conventional credit cards, automobile licenses, and the like. There are, however, some cellular telephone sets which are known as "personal communicators", and which are apparatus so small that they cannot have formed a full size slot and can, accordingly, accommodate only a miniaturized data carrying card, oftentimes vernacularly referred to as a "thumbnail" card. Such "thumbnail" cards are themselves, however, so small in size as to have, say, a width of only about five-eighth inch (5/8") and a length of only about one inch (1"). Because of that small size, it is very easy for such thumbnail cards to become misplaced and difficult to find when carded on the person of the owner of the card. As a result, some cellular telephone customers with personal communicator sets retain their thumbnail cards inserted at all times in the set. That practice, however, compromises the capability of keeping the customer's personal account secure since, if the customer is not watching the set, an opportunity is afforded for the card to be covertly possessed by another long enough to learn from the data carried by the card the personal account number therefor. Thus, the thumbnail cards which must be used with personal communicator sets have the disadvantage that, if separated from the set, they are easily misplaced, but, if not separated from the set, the customer's personal account number may be covertly learned.